The Internet services are currently provided using interconnected long-haul and metro optical networks. In modern long-haul and metro optical networks, optical signals are modulated with digital information and transmitted from one location to another, typically through a length of an optical fiber. To increase the information carrying capacity of the networks, modulated optical signals at different wavelengths, called “wavelength channels”, are grouped together (multiplexed) at one location of the network, transmitted through a common fiber to the other location of the network, and ungrouped (demultiplexed) at the other location.
As the Internet, Voice over Internet Protocol (VoIP) and streamed Internet Protocol (IP) television gain popularity, more and more subscribers desire to access these services from their premises. At present, these services are delivered to individual premises using either a twisted-pair Digital Subscriber Line (DSL) or a coaxial television cable. Due to the increased demand, the DSL and coaxial cable technologies are reaching their information carrying capacity limits, and optical technologies (so-called “Fiber To The Premises”, or FTTP) are increasingly used for delivering Internet services to individual premises.
Most FTTP technologies presently use a passive optical network (PON) architecture to provide fiberoptic access to the premises, because a PON architecture does not require expensive amplification and wavelength selective switching equipment commonly used in long-haul and metro optical networks. To deliver communication services from a central office to multiple individual subscribers, most PON systems use a passive star-type optical splitter and a form of time-division multiplexing (TDM) for delivering downstream and upstream information.
Disadvantageously, TDM-PON systems are quite complex and do not always provide a required degree of security of communications. A wavelength-division multiplexing (WDM) architecture can be attractive for a PON application, because in a WDM-PON, different wavelengths can be assigned to different subscribers or groups of subscribers, thus providing a higher degree of security of communications than a TDM-PON can provide. Furthermore, a WDM-PON architecture can potentially provide a broader bandwidth than a TDM-PON architecture. Nonetheless, WDM-PON systems so far have been relatively costly. For this reason, WDM-PON systems have not yet found a widespread utilization in cost-sensitive FTTH applications.
WDM-PON systems utilize wavelength-selective combiners and splitters of optical signals called “WDM multiplexors” and “WDM demultiplexors”, respectively. To save costs, a WDM multiplexor and a WDM demultiplexor of a WDM-PON system can be combined into a single unit, which is referred to as a “de/multiplexor”. Referring to FIG. 1A, a prior-art arrayed waveguide (AWG) WDM de/multiplexor 100 is shown having a single input port 102 and four output ports 111 to 114. Four wavelength channels λ1C, λ2C, λ3C, λ4C of central (“C”) band of optical communications and four wavelength channels λ1S, λ2S, λ3S, λ4S of short (“S”) band optical communications are present at the input port 102. The WDM de/multiplexor 100 directs wavelengths λ1C, λ1S to the output port 111; wavelengths λ2C, λ2S to the output port 112; wavelengths λ3C, λ3S to the output port 113; and wavelengths λ4C, λ4S to the output port 114. To direct different wavelengths to a same output port, the WDM de/multiplexor 100 uses a diffractive optical device having multiple orders of diffraction. The WDM de/multiplexor 100 is bidirectional, that is, the wavelength channels arriving at the output ports 111-114 can be combined into a single multi-channel signal at the input port 102. Referring now to FIG. 1B, a WDM-PON 120 has two nodes 121 and 122 coupled through a length of an optical fiber 123. Each node 121 and 122 has one WDM de/multiplexor 100. The input ports 102 of the WDM de/multiplexors 100 of the nodes 121 and 122 are connected together by the optical fiber 123. The output ports 111 to 114 of the WDM de/multiplexors 100 are coupled to duplex optical filters 124 coupled to corresponding transmitters 126 and receivers 128. The node 121 uses the wavelength channels λ1C, λ2C, λ3C, λ4C for transmission and the wavelength channels λ1S, λ2S, λ3S, λ4S for reception. The node 122 uses the wavelength channels λ1S, λ2S, λ3S, λ4S for transmission and the wavelength channels λ1C, λ2C, λ3C, λ4C for reception. The direction of flow of the signals is shown with arrows 127. Thus, each WDM de/multiplexor 100 is used for both multiplexing and demultiplexing wavelength channels, whereby significant cost savings can be achieved.
Disadvantageously, in the AWG WDM de/multiplexor 100, and in any diffraction grating based demultiplexor for that matter, the wavelengths of the channels λiS and λiC directed to a same ith output port in different orders of diffraction m and m+1 are tied together by the grating equation: λiS≈λiCm/(m+1) and therefore cannot be selected independently from each other. As a result, the WDM-PON 120 does not allow a system designer to select the wavelength channels λ1C, λ2C, λ3C, λ4C independently from the wavelength channels λ1S, λ2S, λ3S, λ4S. This represents a considerable limitation, especially for a FTTP application where the available bandwidth needs to be utilized to a full extent to provide as broad coverage as possible at a given cost.
It is therefore an object of the invention to provide an optical device for directing and regrouping wavelength channels, wherein the wavelengths of the channels directed to the same output port are independently selectable. The independent wavelength selection improves bandwidth utilization and network efficiency. As a result, a deployment cost to provide a FTTH-based broadband Internet service to individual subscribers is reduced.